Effects of rapid transfer from sea water to fresh water on respiratory variables, blood acid-base status and O₂ affinity of haemoglobin in Atlantic salmon (Salmo salar L.)

Summary

Oxygen consumption, gill ventilation, blood acid-base/ionic status and haemoglobin oxygen affinity were studied in seawater-adapted adult salmon (Salmo salar) during five weeks after transfer into fresh water. Freshwater exposure induced the following changes: Standard oxygen consumption ($\dot M_{O_{ 2} }$) and ventilatory flow ($\dot V_w$) decreased markedly during the first days after transfer, then decreased more gradually until a new steady-state was achieved at which $\dot M_{O_{ 2} }$ and $\dot V_w$ were about 80% and 56% of the control values, respectively. The marked increase in oxygen extraction coefficient (Ew _{O 2}) and the marked decrease in the oxygen convection requirement ($\dot V_w /\dot M_{O_{{\text{ 2}}} }$) were associated with a reduction in the partial pressure of carbon dioxide in arterial blood (Pa _{CO 2}), in spite of a decrease of both ventilatory flow and water CO₂ capacitance. These results suggested that transfer into fresh water induced an increase in branchial diffusive conductance. A biphasic pattern was observed in the time-course of the changes in both plasma ion concentration and acid-base status. During the first 10 days, plasma Na⁺, K⁺, and Cl⁻ concentrations fell abruptly, then more gradually. [Cl⁻] decreased more than [Na⁺] resulting in a progressive increase in the [Na⁺]/[Cl⁻] ratio. During the second phase of acclimation to fresh water plasma Na⁺, K⁺, and Cl⁻ concentrations progressively increased. [Cl⁻] increased more than [Na⁺], so that [Na⁺]/[Cl⁻] ratio decreased. Transfer into fresh water did not significantly change plasma lactate concentration. Upon exposure to fresh water, blood pH increased from 7.94±0.04 to 8.43±0.06 at day 10 and then decreased to 8.08±0.03 at day 34. The increase in blood pH induced by transfer to fresh water initially represented a mixed metabolic/respiratory alkalosis. However, after 15 days Pa _{CO 2} had returned to pretransfer values and the alkalosis was purely metabolic. The metabolic component of the alkalosis was associated with appropriate changes in the plasma strong ion difference (S.I.D.). Blood alkalosis moved the oxygen dissociation curve to the left, so that P₅₀ was decreased by 30% below the value in seawater for the maximal increase in blood pH. This rise in haemoglobin affinity for O₂, associated with a marked increase in blood buffer capacity, are regarded as adaptative processes allowing the salmon to cope with the markedly increased energy expenditure required for upstream migration.